The present invention relates to a photoelectronic device (semiconductor optical module) and a method of manufacturing the same and, more particularly, to a technique effective for bonding and securing an optical fiber to a silicon substrate having a groove on the surface thereof referred to as xe2x80x9csilicon platformxe2x80x9d using a bonding element such as thermosetting resin or an ultraviolet-setting adhesive.
Photoelectronic device incorporating a semiconductor laser (semiconductor laser chip) are used as light sources for information processing apparatuses and light sources for optical communication.
One well-known type of photoelectronic devices is photoelectronic devices (semiconductor laser devices) having a box-type package structure.
Referring to passive alignment mounting utilizing a silicon platform, for example, passive alignment type optical modules are known which has a structure in which a silicon platform is mounted in a package having leads and a cover; a laser diode, a monitor photodiode and a pig-tail optical fiber are mounted on the silicon platform; and a presser plate is mounted.
The inventors are working on techniques for securing an optical fiber on a silicon platform in a short period of time and techniques to reduce the cost of packaging in optoelectonic apparatuses incorporating a semiconductor laser (passive alignment type optical module).
The inventors made the following studies of techniques for securing an optical fiber on a silicon platform in a short period of time.
In a conventional semiconductor optical module utilizing a silicon platform (support substrate), an optical fiber embedded in the silicon platform is secured after adjusting optical coupling between the end of the optical fiber embedded in a groove on the silicon platform so as to trail along it and a semiconductor laser chip secured on the surface of the silicon platform. It is secured using (1) a technique for securing it with thermosetting resin (thermosetting epoxy resin) or an adhesive such as an ultraviolet-setting adhesive and (2) a technique for securing the optical fiber while pressing it against the silicon platform with a presser plate.
When an optical fiber is secured with thermosetting resin, a process of applying and setting the thermosetting resin must be performed with the optical fiber pressed against the silicon platform to remain static after adjustment of optical coupling.
However, this method reduces the efficiency of an operation of securing optical fiber because the thermosetting resin takes a long time to be set. For example, in the case of epoxy resin used as thermosetting resin, the setting process takes about two minutes even at a temperature of 150xc2x0 C. which is in the excess of the guaranteed temperature for an optical fiber.
Since the adjustment of optical coupling is performed using a fiber inserting apparatus, the long time spent for the adjustment of optical coupling results in a reduction in the operating efficiency of the fiber inserting apparatus. In addition, a fiber inserting apparatus is expensive and consequently increases the cost for the adjustment of optical coupling.
The method of setting thermosetting resin on a fiber inserting apparatus after the adjustment of optical coupling has had a problem in that the process of setting thermosetting resin can not be performed on a batch process basis and this reduces the operating efficiency of a fiber inserting apparatus further.
The efficiency of the conventional operation of securing an optical fiber with thermosetting resin is thus reduced, which hinders any reduction in the manufacturing cost of a photoelectronic device (optical module).
The conventional technique for securing an optical fiber with thermosetting resin results in a reduction of the yield of optical axis alignment because the state of optical coupling can change if the optical fiber is moved before the thermosetting resin is reliably set.
Referring to the technique of securing an optical fiber on a silicon platform by applying an ultraviolet-setting adhesive to a part of the optical fiber and silicon platform and thereafter irradiating the ultraviolet-setting adhesive with ultraviolet light to set the ultraviolet-setting adhesive, it secures an optical fiber with reduced reliability because regions which can not be irradiated with ultraviolet light are not set, although the setting process utilizing ultraviolet irradiation allows an optical fiber to be secured in a short period of time.
A possible solution is a two-step processing mode which involves setting by means of irradiation with ultraviolet beams and heat setting using an ultraviolet-setting adhesive which can be set by both ultraviolet beams and heat. In this case, the efficiency of an operation of securing an optical fiber (turnaround time: TAT) is reduced because the setting process using heat takes time. An example of this type of ultraviolet-setting adhesives takes a heating time as long as 60 minutes at 120xc2x0 C.
A heat setting process at a processing temperature as high as 120xc2x0 C. and with a long processing time as described above can result in deterioration of resin covering an optical fiber (the region of a fiber cable).
In a structure in which a metalized layer is provided on the surface of an optical fiber comprising a core and a clad (optical fiber core) and in which the metalized layer is used to secure the optical fiber to a silicon platform or a cylindrical fiber guide for guiding the optical fiber with solder, when an optical fiber is fitted in a groove on a silicon platform so as to trail along it, variation of the thickness of the metalized layer can make it difficult to adjust optical coupling between the core of the optical fiber and a semiconductor laser chip.
Under such circumstances, the inventors are studying a technique as described below for securing an optical fiber on a silicon platform, although it is not a known technique. Specifically, in a conventional method in which a silicon platform (support substrate) having a groove on the surface thereof is prepared; a photoelectric conversion element (semiconductor laser chip) is thereafter secured on the surface of the support substrate at one end of the groove; an optical fiber is fitted in the groove so as to trail along it; and, thereafter, the state of transmission and reception of light between the photoelectric conversion element and the optical fiber is adjusted and the optical fiber is secured on the support substrate with thermosetting resin, according to the technique, the optical fiber is preliminaryly secured using securing means in a securing time shorter than the setting time of the thermosetting resin while it is pressed against the support substrate, and is thereafter finally secured with thermosetting resin with the pressing cancelled.
For example, an ultraviolet-setting adhesive is applied to a part of the optical fiber and support substrate; the ultraviolet-setting adhesive is set by irradiating it with ultraviolet light to preliminary secure the optical fiber on the support substrate; and a part of the optical fiber which is farther from the semiconductor laser chip than the preliminary secured position is covered with thermosetting resin.
According to this technique, since preliminary securing is carried out using an ultraviolet-setting adhesive, a support substrate and the like can be moved even after the application of thermosetting resin and before the thermosetting resin is set. This allows the support structure and the like to be removed from a fiber inserting apparatus in a short period of time, and the process of setting the thermosetting resin (final securing) can therefore be carried out on a batch process basis. Such a batch process makes it possible to reduce the time required for securing an optical fiber on one support substrate. The reliability of optical coupling is also improved.
In addition to the employment of this technique, the inventors also studied techniques for reducing the cost of a package. In order to achieve a reduction in the package cost, they decided to make a package main body (case) and a cover element (cap) forming a package from plastic and to adopt a structure in which the case and cap are bonded with resin. Further, since plastic is less resistant to humidity than ceramics, it was conceived to improve humidity resistance by sealing the case with silicone gel to cover the surface of components on the support structure including a semiconductor laser chip.
Referring to this technique, however, it was revealed by the inventors that such a silicone gel sealing structure reduces the strength and reliability of the securing of an optical fiber and also reduces humidity resistance. It was found that this is attributable to bubbles generated in silicone gel.
Experiments and studies made on the mechanism of the generation of bubbles revealed that the number of bubbles can increase from the initial value depending on the temperature cycle, i.e., the temperature of the environment of use.
FIG. 25 is a schematic view of a region in which an optical fiber 3 is secured in a groove 2 on a silicon platform (support substrate) 1 through preliminary securing with an ultraviolet-setting adhesive 4 and final securing with thermosetting resin 5 and in which the upper surface of the silicon platform 1 is covered by silicone gel 6. The optical fiber 3 is formed by a clad 3b and a core 3a located in the center of the same. The two-dot chain line represents a semiconductor laser chip 6. As shown in FIG. 25, the generation of a bubble 10 is likely to occur in the silicone gel in an enclosed region 9 defined by the surface of the groove 2 of the silicon platform 1 and the optical fiber.
The presence of the bubble reduces the strength and reliability of the securing of the optical fiber 3 to the silicon platform 1.
Humidity resistance is reduced not only by the presence of the bubble 10 itself but also by the fact that the region of the bubble acts as a nucleus to trap any invasive moisture to make it difficult to release the moisture to the outside. A semiconductor laser chip, light-receiving element and the like are provided ahead of the end of the optical fiber and a wiring layer, wires and the like are provided around the same. Therefore, any moisture trapped by the bubble 10 can cause oxidation and corrosion of those parts to reduce the humidity resistance of the optical module.
With moisture trapped at the region of a bubble, the moisture can be frozen when the optical module is exposed to a temperature below the freezing point, which can cause troubles attributable to a resultant change in the volume.
As shown in FIGS. 26A, 26B, 27A and 27B, an experiment was conducted in which a metal frame 16 was placed on the bottom of a container 15; two capillaries 17 made of glass (having an inner diameter of 0.13 mm) were arranged thereon in parallel and in contact with each other; and the interior of the container 15 was filled with silicone gel 6 to cover the surface and interior of the capillaries 17 such that no bubble was involved. Thereafter, the container 15 was kept under certain curing process conditions (a processing temperature of 120xc2x0 C. and a processing time of 60 minutes). FIGS. 26A and 26B are schematic views showing the distribution of bubbles 10 in the silicon gel set under the curing process conditions. FIG. 26A is a plan view, and FIG. 26B is a sectional view.
After the silicone gel was set, environmental tests such as temperature cycles were conducted. Specifically, (1) 40 cycles of about 35 minutes at a temperature in the range from xe2x88x9240 to +85xc2x0 C., (2) 136 hours at a high temperature and humidity (a temperature of 85xc2x0 C. and a relative humidity of 85%), (3) high temperature baking (120xc2x0 C.) for 30 minutes and (4) storage at a low temperature (xe2x88x9255xc2x0 C.) for 1.5 hours were carried out in the order listed. FIGS. 27A and 27B are schematic views showing the distribution of bubbles 10 generated in the silicon gel during the environmental tests including temperature cycles. FIG. 27A is a plan view, and FIG. 27B is a sectional view.
The bubbles 10 in FIGS. 26A, 26B, 27A and 27B are illustrations based on photographs which represent accurate positions, although the shapes of the bubbles may be slightly different from the real ones.
As shown in FIGS. 26A and 26B, the bubbles 10 are dispersed across the inner diameter of the capillaries 17, but there is no bubble at both ends of the capillaries 17. The reason is assumed to be the fact that the silicone gel can freely move in and out the capillary 17 at both ends thereof (open regions), and it is assumed that cavities or bubbles 10 are generated at inner diameter regions deep in the capillaries 17 because the movement of silicone gel in such regions is not sufficient to compensate for a reduction of the volume attributable to contraction.
Further, as shown in FIGS. 27A and 27B, since the capillaries are repeatedly exposed to varying temperature and humidity during the environmental tests, new cavities are generated as the silicone gel moves to increase bubbles 10. It is assumed that the shapes of bubbles 10 change as a result of integration or separation of cavities adjacent to each other. Bubbles had greater configurations and were subjected to great positional shifts at a high temperature of 120xc2x0 C., and many small bubbles were generated at a low temperature of xe2x88x9255xc2x0 C.
FIGS. 27A and 27B show that new bubbles 10 were generated in a region where no bubble 10 had existed as shown in FIGS. 26A and 26B, specifically, the region surrounded by the metal frame 16 and the two capillaries 17 (the enclosed region 9).
It was found that when the interior of the plastic case was sealed with silicone gel to over the surface of components on the support substrate 1 including the semiconductor laser chip 7, bubbles 10 might be generated not only in the silicon gel 6 filled in the groove 2 under the optical fiber 3 as shown in FIG. 28 but also between the end face (front incidence surface) of the optical fiber 1 and the semiconductor laser chip 7.
The reason is assumed to be the fact that the gap between the end face of the optical fiber 3 and the front emission surface of the semiconductor laser chip 7 does not act as an open region because it is as small as 40 to 50 xcexcm and that the gap is likely to generate bubbles when heated repeatedly. Specifically, while no bubble 10 was observed at the gap between the end face of the optical fiber 3 and the front emission surface of the semiconductor laser chip 7 at an early stage when the silicone gel 6 had been filled and set after assembly, the phenomenon of generation of bubbles 10 at the gap between the end face of the optical fiber 3 and the front emission surface of the semiconductor laser chip 7 occasionally occurred after the heat cycle test.
When a bubble 10 is generated at the gap between the end face of the optical fiber 3 and the semiconductor laser chip 7 to come into the optical path of laser light 11 emitted from the emission surface of the semiconductor laser chip 7 (see FIGS. 29 and 30), since the bubble 10 acts as an lens, the direction of the laser light 11 emitted by the semiconductor laser chip 7 is changed (eclipsed) to disallow optical coupling to the optical fiber 3 or to reduce the efficiency of optical coupling. When the optical fiber 3 is a single mode fiber whose core 3a has a diameter as small as about 10 xcexcm, optical coupling is often disabled. Reference number 31 in FIGS. 28 through 30 represents a light-receiving element 31 for receiving the laser light 11 emitted from the rear emission surface of the semiconductor laser chip 7. In FIG. 30, the silicone gel 6 is present on the entire upper surface of the support substrate 1.
It is an object of the invention to provide a photoelectronic device with high optical coupling efficiency and a method of manufacturing the same.
It is another object of the invention to provide a photoelectronic device in which an optical fiber is secured with high strength and reliability and a method of manufacturing the same.
It is still another object of the invention to provide a photoelectronic device having excellent humidity resistance and a method of manufacturing the same.
It is still another object of the invention to provide a photoelectronic device in which an optical fiber can be secured in a shorter working time and a method of manufacturing the same.
It is still another object of the invention to provide a photoelectronic device which can be manufactured at a reduced cost and a method of manufacturing the same.
The above and other objects and novel features of the invention will become apparent from the description of this specification and the accompanying drawings.
Typical aspects of the invention disclosed here can be briefly described as follows.
(1) There is provided a photoelectronic device comprising a support substrate (silicon platform) constituted by a mounting portion for mounting a photoelectric conversion element (semiconductor laser chip) on one surface thereof and a silicon substrate having a groove for guiding an optical fiber extending toward the mounting portion, a photoelectric conversion element secured on the mounting portion and an optical fiber fitted in the groove at one end thereof and secured on the support substrate at regions excluding the utmost end thereof, wherein the optical fiber fitted in the groove is secured with a first bonding element injected to fill the groove under the optical fiber for preliminary securing the optical fiber on the support substrate and a second bonding element for finally securing the optical fiber on the support substrate while covering a part of the optical fiber and support substrate and wherein a protective element transparent to light transmitted by the optical fiber covers a region including the photoelectric conversion element on one surface of the support substrate and one end of the optical fiber. The second bonding element covers all or a part of the region where the first bonding element exists. The first bonding element is constituted by an ultraviolet-setting adhesive, and the second bonding element is constituted by thermosetting resin. The support substrate is secured in a case made of plastic having a guide for guiding the optical fiber. The case is filled with the protective element to cover the support substrate, photoelectric conversion element, optical fiber and the like. The case is closed with a cap made of plastic and is secured on the support substrate with an adhesive. The protective element is constituted by any of silicone gel, silicone rubber, low-stress epoxy resin, acrylic resin or urethane resin. For example, it is constituted by silicone gel. With this configuration, bubbles in sizes equal to or greater than one half of the distance between the two points of the groove in contact with the circumferential surface of the optical fiber are not present in the region defined by the optical fiber and groove and the region between one end face of the optical fiber and the semiconductor laser chip.
This configuration is characterized by the preliminary securing and final securing referred to as xe2x80x9cfirst securingxe2x80x9d and xe2x80x9csecond securingxe2x80x9d, respectively. In a certain limited aspect, it may be stated that the optical fiber is secured on the support substrate using first and second securing techniques (means) having different securing speeds and that the securing speed of the first securing means is higher than that of the second securing means.
Such a photoelectronic device is manufactured according to the following method.
The method comprises the steps of:
providing a support substrate having a photoelectric conversion element mounted thereon and having a groove for guiding an optical fiber extending toward the photoelectric conversion element;
applying an ultraviolet-setting adhesive to a part of the groove on the support substrate, fitting one end of the optical fiber in the groove on the support substrate and adjusting optical coupling between the photoelectric conversion element and optical fiber with the groove under the optical fiber filled with the ultraviolet-setting adhesive;
preliminary securing the optical fiber on the support substrate by irradiating the ultraviolet-setting adhesive with ultraviolet light to set it; and
covering a part of the optical fiber and support substrate with thermosetting resin and setting the thermosetting resin to finally secure the optical fiber on the support substrate.
Specifically, it is a method of manufacturing a photoelectronic device comprising:
a package constituted by a case made of plastic having a guide for guiding an optical fiber and a cap made of plastic for closing the case, attached to the case with an adhesive;
a support substrate secured in the case having a photoelectric conversion element mounted on one surface thereof and having a groove for guiding an optical fiber extending toward the photoelectric conversion element and an optical fiber guided by the guide into and out of the package, wherein one end of the optical fiber extending in the package is fitted in the groove on the support substrate and is secured on the support substrate through preliminary securing with an ultraviolet-setting adhesive and final securing with thermosetting resin. The method comprises the steps of:
applying the ultraviolet-setting adhesive to a part of the groove on the support substrate, fitting one end of the optical fiber in the groove on the support substrate and adjusting optical coupling between the photoelectric conversion element and optical fiber with the groove under the optical fiber filled with the ultraviolet-setting adhesive;
preliminary securing the optical fiber on the support substrate by irradiating the ultraviolet-setting adhesive with ultraviolet light to set it;
covering a part of the optical fiber and support substrate with thermosetting resin and setting the thermosetting resin to finally secure the optical fiber on the support substrate; and
filling the case with a protective element transparent to light transmitted by the optical fiber before mounting the case and setting the same. The protective element is constituted by any of silicone gel, silicone rubber, low-stress epoxy resin, acrylic resin or urethane resin. For example, silicon gel is used. Securing is carried out by determining positions for the preliminary securing and/or final securing such that all or a part of the preliminary securing portion is covered by the final securing portion. The process of setting the thermosetting resin at the final securing is performed as a batch process.
A structure may be employed in which an optical fiber is secured on a support substrate with only a first bonding element. Specifically, there may be provided a photoelectronic device comprising a support substrate constituted by a mounting portion for mounting a photoelectric conversion element on one surface thereof and a support substrate having a groove for guiding an optical fiber extending toward the mounting portion, a photoelectric conversion element secured on the mounting portion and an optical fiber fitted in the groove and secured on the support substrate at regions excluding the utmost end thereof, the device having a structure wherein the optical fiber fitted in the groove is secured with a first bonding element injected to fill the groove under the optical fiber for securing the optical fiber on the support substrate and wherein a protective element transparent to light transmitted by the optical fiber covers a region including the photoelectric conversion element on one surface of the support substrate and one end of the optical fiber. In this case, the first bonding element is constituted by an ultraviolet-setting adhesive or thermosetting resin. Thus, after the ultraviolet-setting adhesive is applied to the groove on the support substrate, optical coupling between the photoelectric conversion element and optical fiber is adjusted with the groove under the optical fiber filled with the ultraviolet-setting resin.
(2) In the configuration described in the aspect (1), the support substrate, photoelectric conversion element and the end of the optical fiber are covered by a package constituted by insulating resin formed by molding resin, and the protective element is provided in the package to block the path of moisture that enters the photoelectric conversion element from the outside of the package.
According to the aspect (1), (a) while the package is formed by a case and a cap made of plastic, humidity resistance can be improved because the case is filled with silicone gel.
(b) When the optical fiber is fitted in the groove on the silicon platform (support substrate), a space is defined by the groove under the optical fiber. This space is filled with the ultraviolet-setting adhesive. Therefore, the silicone gel does not enter the region under the optical fiber associated with the preliminary securing portion when the case is filled with the silicone gel before sealing with the cap, and no bubble is caused by the setting and contraction of the silicone gel. This makes it possible to prevent any reduction in the strength and reliability of the securing of the optical fiber 3 attributable to bubbles and to prevent problems such as freezing of moisture trapped by bubbles.
Specifically, even if moisture enters from the outside along the optical fiber, since the gap between the optical fiber and groove at the preliminary securing portion is filled with the ultraviolet-setting adhesive for preliminary securing, the invasion of moisture is prevented at the preliminary securing portion, and there is no nucleus like a bubble in the silicone gel that can trap moisture. This prevents trapping of moisture to improve humidity resistance and eliminates the possibility of freezing of moisture during use at a low temperature.
In the structure in which the optical fiber is secured on the support substrate using only a first bonding element constituted by an ultraviolet-setting adhesive or thermosetting resin, since the groove under the optical fiber is filled with the first bonding element, the silicone gel does not enter the groove region under the optical fiber, and this also prevents the generation of bubbles attributable to the setting and contraction of the silicone gel.
Further, only a small amount of silicone gel enters the groove under the end portion of the optical fiber because the end portion protruding from the region secured using the first bonding element is as short as several hundred xcexcm, and a region open to the atmosphere exists ahead the end of the optical fiber. Thus, the silicone gel moves when it sets and contracts, which suppresses the generation of bubbles. This not only prevents the generation of bubbles in the silicone gel at the end of the optical fiber and under the same to eliminate nuclei to trap moisture but also eliminates bubbles from the gap between the semiconductor laser chip and optical fiber. This prevents eclipse of laser light attributable to bubbles to allow optical coupling of the optical fiber with high efficiency.
Even if bubbles are generated in the silicone gel in the groove under the optical fiber, such bubbles have small diameters.
(c) After the adjustment of optical coupling, the optical fiber is preliminary secured to the region of the groove on the silicon platform with the ultraviolet-setting adhesive and is thereafter subjected to final securing with the thermosetting resin. This improves the reliability of optical coupling.
(d) Since the thermosetting resin has high bonding strength, the optical fiber is reliably secured to the silicon platform through the final securing using the thermosetting resin, and the optical fiber is thus secured with improved reliability. The optical fiber is secured at the preliminary securing portion such that the optical coupling between the optical fiber and the semiconductor laser chip is not deteriorated, and the final securing portion improves the securing strength of the optical fiber.
(e) Since preliminary and final securing is carried out as in (c) and (d) above, the optical fiber is secured on the silicon platform with high optical coupling and high reliability of coupling.
(f) When the optical fiber is secured in the groove on the silicon platform, the optical fiber is pressed against the silicon platform after optical coupling between the semiconductor laser chip and optical fiber is adjusted, and the optical fiber is preliminary secured in such a state by applying the ultraviolet-setting adhesive and irradiating the ultraviolet-setting adhesive with ultraviolet light to set it. This makes it possible to reduce the time required for the preliminary securing to several tens seconds.
(g) Since the preliminary securing using the ultraviolet-setting adhesive provides high securing reliability in a short term, the optical coupling between the optical fiber and semiconductor laser chip is not deteriorated during the time interval before the subsequent final securing. Therefore, when the optical fiber is finally secured with the thermosetting resin (epoxy resin) thereafter, the thermosetting process following the application of the thermosetting resin can be carried out on a batch process basis. This makes it possible to improve the efficiency of the operation of securing the optical fiber, thereby achieving a reduction in the manufacturing cost of the photoelectronic device.
(h) The preliminary securing using the ultraviolet-setting adhesive is carried out on a fiber inserting apparatus for aligning the optical axes of the semiconductor laser chip and optical fiber. Since the time for the preliminary securing of the optical fiber is reduced (to several tens seconds), the operating efficiency of the fiber inserting apparatus can be improved.
(i) Since a fiber inserting apparatus is expensive, improved operating efficiency of a fiber inserting apparatus results in a reduction of the manufacturing of the photoelectronic device.
In the aspect (2) described above, there is the following effect in addition to the effects according to the aspect (1). In this aspect, it is possible to achieve high productivity and to reduce the manufacturing cost of a photoelectronic device because the package is formed by molding insulating resin.